Since the most dramatic instances of speciation seem to have happened in the aftermath of mass extinctions, this essay will survey extinction first. A corollary to is that if any critical nutrient falls low enough, the nutrient deficiency will not only limit growth, but the organism will be stressed. If the nutrient level falls far enough, the organism will die. A human can generally survive between one and two months without food, ten days without water, and about three minutes without oxygen. For nearly all animals, all the food and water in the world are meaningless without oxygen. Some microbes can switch between aerobic respiration and fermentation, depending on the environment (which might be a very old talent), but complex life generally does not have that ability; nearly all aerobic complex life is oxygen dependent. The only exceptions are marine life which has adapted to . Birds can go where mammals cannot, , for instance, or being , due to their . If oxygen levels rise or fall very fast, many organisms will not be able to adapt, and will die.
In South America, its animals continued to evolve in isolation, and some huge ones appeared. In the Miocene, the flew in South American skies; it looked like a giant condor, had a seven-meter wingspan, and weighed 70 kilograms. The lived in South America in the late Miocene and early Pliocene. first appeared, as well as a , and some inhabited the large herbivore guild and looked like guild members on other continents, for another instance of convergent evolution. In Australia, the Miocene fossil record is thin, but recent findings demonstrate that all Miocene mammals were marsupials, except for bats. Kangaroos diversified into different niches; some were rat-sized and others became carnivorous. foraged in the Miocene, and marsupial lions first appeared in the Oligocene, kept growing over the epochs, and when humans arrived about 50 kya, they were . , as they still did in South America, although just how carnivorous some may have been is debated.
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Scientists are unanimous that the Western Hemisphere’s indigenous peoples primarily came from East Asia, but there has been a cottage industry for centuries proposing other ideas. When Thomas Jefferson sent the expedition to North America’s west coast in 1804 to , they were alerted to find the lost tribes of Israel. But genetic, anatomical, archeological, and other evidence has long since settled the issue of where American Indians came from, and by far the leading hypothesis is that humans migrated to North and South America beginning about 15 kya, and there may have been a migration along the Pacific coastline, which continued the . As the , a corridor between them formed and humans walked to North America about 11 kya. Those arrivals founded the . The sudden disappearance of virtually all the megafauna of North and South America followed those humans, particularly those that came by land and spread. That situation is where the original “” label was used.